First principles study on silicon nanostructures as potential anode materials for Li-ion batteries.
Date of Issue2013
School of Materials Science and Engineering
School of Materials Science & Engineering
Rechargeable lithium-ion (Li-ion) batteries are the popular power sources in most of portable electronic devices, power tools and electric vehicles (EVs). Moreover, Li-ion batteries play an important role in the development of clean and efficient energy, serving as prominent energy storage devices for renewable energy sources, such as solar and wind. The emerging applications greatly raise the level of demands to batteries performance, however, existing Li-ion battery technology is reaching its limit, due to its relatively low Li storage capacity. The ever-growing demand for next-generation batteries requires electrode materials with larger capacity and higher power density, faster ionic diffusion and electronic transfer, lower cost and environmental friendliness. Silicon is among the promising novel anode materials for Li-ion batteries. The specific capacity of Si is an order of magnitude higher than that of conventional graphite anode, but the large volume expansion of bulk Si during lithiation and subsequent poor cycling behavior hinder its commercial use. The use of Si nanostructures was shown to be able to provide the means to overcome the abovementioned challenges. This thesis aims at the computational design and evaluation of novel Si-based nanostructures for their potential use in Li-ion battery anodes. Firstly, we investigate silicon cluster/carbon nanotube (Si/CNT) hybrid nanostructures, where CNTs serve as a buffer and mechanical support for Si. The strength of Si/CNT interface is at the main focus here, in order to maintain electric contact and prevent Si particles from detachment and agglomeration. Since the interaction of Si cluster with pristine CNT is relatively weak, the functional groups (linkers) are introduced, aimed to enhance Si-CNT binding. We systematically evaluate the effects of the main components (i.e. Si cluster, CNT support and linker) on the properties of hybrid system, such as morphology, interfacial bonding and electronic structure. From our calculations, we determine a suitable design strategy of Si/CNT hybrids, which not only increases the binding strength between Si clusters and CNT by 3 times under normal conditions, but also greatly contributes to the stability of hybrid material during lithiation. Secondly, we theoretically study the properties of novel Si nanotubes of two structural types – hexagonal and gearlike. From our calculations, SiNTs show higher reactivity toward the adsorption of Li adatoms than CNTs and Si nanoclusters. Considering the importance of Li kinetics, we demonstrate that the interior of SiNTs may serve as a fast Li diffusion channel. The important advantage of SiNTs over its carbon analogues is 7 times reduction of energy barrier for the sidewall penetration of Li atoms into the nanotube interior. The improvements are attributable to the large void spaces, serving as fast Li diffusion pathways. This prepossesses the easier Li diffusion inside the tube and subsequent utilization of interior sites, enhancing the Li storage capacity of the system. Finally, we use first principles calculations to investigate novel ultrathin silicon nanosheets (SiNSs) with a thickness of only few nanometres. Calculations show that binding energy of Li shows a strong dependency on the thickness of Si nanosheets; meanwhile, in all cases the surface sites are the most energetically favorable for Li insertion. Most importantly, our results show that Li diffusion on the surfaces of Si nanosheets is very fast (the activation barrier is more than 0.3 eV smaller on the surface than in the bulk). In addition, Li diffusion in nanosheets is very sensitive to their surface chemistry (e.g. passivation with hydrogen or halogens). Considering the high surface-to-volume ratio of nanosheets and fast Li surface diffusion, these results show a great potential of these nanostructures as electrode materials for the next-generation Li-ion batteries. In conclusion, detailed computational studies are performed which may serve as guidelines for the development of Si nanostructured anodes in the future. Novel nanostructures are investigated, and the correlation between structural features, dimensional effects, surface functionalization and Li insertion characteristics are established. This study demonstrates the viability of using computations to design novel electrode materials with improved properties. Similar computational methodology can be applied in the future studies of alternative electrode materials and other battery chemistries, such as sodium-ion and magnesium-ion.
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